that most index heads have a 40 to 1 ratio. One
well-known exception has a 5 to 1 ratio (see fig.
7-14). This ratio is made possible by a 5 to 1 gear
ratio between the index crank and the dividing head
spindle. The faster movement of the spindle with one
turn of the index crank permits speedier production.
It is also an advantage when you true work or test it
for run out with a dial indicator. While this dividing
head is made to a high standard of accuracy, it does
not permit as wide a selection of divisions by simple
indexing. Later in this chapter, we'll discuss
differential indexing that you can do on the 5 to 1 ratio
dividing head by using a differential indexing
attachment.
The dividing head (also called an index head)
may also be geared to the lead screw of the milling
machine by a driving mechanism to turn the work-as
required for helical and spiral milling. The index
head may have one of several driving mechanisms.
The most common of these is the ENCLOSED
DRIVING MECHANISM, which is standard
equipment on some makes of plain and universal knee
Figure 7-15.--Enclosed driving mechanism.
and column milling machines. The enclosed driving
mechanism has a lead range of 2 1/2 to 100 inches and
is driven directly from the lead screw.
A = Gear on the worm shaft (driven)
B = First gear on the idler stud (driving)
Figure 7-15 shows the gearing arrangement used
on most milling machines. The gears are marked as
C = Second gear on idler stud (driven)
follows:
D = Gear on lead screw (driving)
E and F = Idler gears
LOW LEAD DRIVE: For some models and
makes of milling machines a low lead driving
mechanism is available; however, additional parts
must be built into the machine at the factory. This
driving mechanism has a lead range of 0.125 to 100
inches.
LONG and SHORT LEAD DRIVE: When an
extremely long or short lead is required, you can use
the long and short lead attachment (fig. 7-16). As
with the low-lead driving mechanism, the milling
machine must have certain parts built into the
machine at the factory. In this attachment, an
auxiliary shaft in the table drive mechanism supplies
power through the gear train to the dividing head. It
also supplies the power for the table lead screw, which
Figure 7-14.--Universal spiral dividing head with a 5 to 1
is disengaged from the regular drive when the
ratio between the spindle and the index crank.
attachment is used. This attachment provides leads in
the range between 0.010 and 1000 inches.
7-10